UFC 3-260-03

15 Apr 01

I

15,000

263,084

86/R/D/W/T

(580,000)

II

3,000

263,084

102/R/D/W/T

(580,000)

III

500

263,084

126/R/D/W/T

(580,000)

IV

100

263,084

159/R/D/W/T

(580,000)

(2) Determine the period of thaw weakening. The maximum load of 263,084 kg (580,000 lb) can be

applied throughout the year.

18. EXAMPLE 3. Evaluate an Army Class IV airfield pavement taxiway consisting of 102 millimeters

(4 inches) of asphalt concrete 152 millimeters (6 inches) of crushed aggregate base, and 508 millimeters

(20 inches) of subbase on a silty sand subgrade. The pavement is in fair condition. The evaluation will be

for the C-130 aircraft at a design pass level of 26,000. The aircraft traffic is applied uniformly throughout

the year. The subgrade has a PI of 10, a dry density of 1,602 kilograms/cubic meter (100 pcf) and an

average water content of 18 percent. From field tests, the subgrade CBR during the normal period was 7.

The base was a nonfrost-susceptible well-graded gravel (GW) with a normal CBR value of 80, and the

subbase was a nonfrost-susceptible sandy gravel (GP) and exhibited a normal period CBR value of 50.

The average dry unit weight and average water content of the base/subbase layer are

2,163 kilograms/meters (135 pcf) and 3 percent, respectively. The highest ground water is at the sub-

grade surface. For this example, this airfield is in Fairbanks, AK.

a. From WORLDINDEX:

DFI = 86,496 EC hours (6,487 EF days)

Mean annual temperature = -3 EC (26.6 EF)

Mean freezing length = 188 days

Mean start of freezing occurs on October 11

Mean end of freezing occurs on April 16

b. Determine if Base/Subbase and/or Subgrade is Frost Susceptible. From table 7-1, the subgrade

is classified as a F4 frost-susceptible soil.

c. Determine Depth of Frost Penetration. The depth of frost penetration (d) estimated from the

MODBERG program is 2.5 meters (100 inches).

d. Evaluate for Complete Frost Penetration. The combined base thickness of pavement and base

to prevent any freezing of the subgrade in the design freezing index year (complete protection) is

2.5 meters (100 inches). With a 102-millimeter-(4-inch-) thick pavement, the thickness of base course

(c) for zero penetration of the subgrade is 2.2 meters (96 inches). The thickness of the pavement, base,

and subbase layer (x) is 76 millimeters (30 inches). Since x # c, then the pavement structure was not

designed for complete frost protection.

e. Evaluate for Limited Subgrade Frost Penetration. The ratio of subgrade to base course water

content r = 18/3 = 6. From figure 7-5, using the maximum permissible ratio r of 2.0 applicable to traffic

7-13